Fix compile with two gflags

Summary:
If the system has gflags with both `google` and `gflags` namespaces installed, we try to define GFLAGS as two things. This breaks the compile.

Fix: Use `else if` -- try compiling with `google` namespace only if compile with `gflags` failed

Test Plan: build_tools/build_detect_platform correctly identifies gflags

Reviewers: lgalanis

Reviewed By: lgalanis

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D37389

build_tools/build_detect_platform

@@ -200,29 +200,19 @@ EOF
         JAVA_LDFLAGS="$JAVA_LDFLAGS -lsnappy"
     fi
 
-
     # Test whether gflags library is installed
     # http://code.google.com/p/gflags/
     # check if the namespace is gflags
-    $CXX $CFLAGS -x c++ - -o /dev/null 2>/dev/null  <<EOF
-      #include <gflags/gflags.h>
-      using namespace gflags;
-      int main() {}
-EOF
-    if [ "$?" = 0 ]; then
+    if echo "#include <gflags/gflags.h>\nusing namespace gflags;\nint main() {}" | \
+      $CXX $CFLAGS -x c++ - -o /dev/null 2>/dev/null; then
         COMMON_FLAGS="$COMMON_FLAGS -DGFLAGS=gflags"
         PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lgflags"
-    fi
-
-    # check if namespace is google
-    $CXX $CFLAGS -x c++ - -o /dev/null 2>/dev/null  <<EOF
-      #include <gflags/gflags.h>
-      using namespace google;
-      int main() {}
-EOF
-    if [ "$?" = 0 ]; then
-        COMMON_FLAGS="$COMMON_FLAGS -DGFLAGS=google"
-        PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lgflags"
+    else
+      if echo "#include <gflags/gflags.h>\nusing namespace google;\nint main() {}" | \
+        $CXX $CFLAGS -x c++ - -o /dev/null ; then
+          COMMON_FLAGS="$COMMON_FLAGS -DGFLAGS=google"
+          PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -lgflags"
+      fi
     fi
 
     # Test whether zlib library is installed